The present invention is directed to a thin-film optical interference filter designed and manufactured to have high-transmittance of blue light and high reflectance of infrared light.
Optical interference filters of the long-wave pass and short-wave pass types are normally based upon quarterwave stacks. See, for example, H. A. Macleod, "Thin-film Optical Filters," American Elsevier, New York (1969).
A quarterwave stack generally consists of alternating layers or "series" of high-reflectance materials and low-reflectance materials. See, for example, A. Thelen, "Equivalent Layers in Multilayer Filters," J. Opt. Soc. Am., 50:1533-1538 (1966). See also U.S. Pat. No. 2,412,496, issued Dec. 10, 1946, to Dimmick.
One problem typically encountered in the use of optical filters based upon quarterwave stacks is known as a "halfwave hole." The halfwave holes appear as dips or minima in the transmittance curve, occurring especially at incident angles approaching 50.degree.. See, H. A. Macleod, "Performance Limiting Factors in Optical Coatings," Proceedings of the Los Alamos Conference on Optics, '81, D. H. Liebenberg, ed., SPIE, 288:580-586 (1981).
The nature of the halfwave holes is such that monitoring errors, which cause departure from the strict halfwave thickness of the layers, invariably lead to transmittance dips in the transmission band. No satisfactory compensation for wide-angle filters has been discovered for this error, which must be kept as small as possible.
Another cause of a halfwave hole is layer dispersion. A layer which appears as a quarterwave at 900 nm does not necessarily appear as a halfwave at 450 nm. This is due to the fact that the film's refractive index may increase at shorter wavelengths. Since the variation in refractive index is generally less for low-index layers, this variation causes a distinction between the wavelength for which the high-index layers are halfwaves from that for which the low-index layers are halfwaves. Thus, the high and low-index layers are not halfwaves at the same wavelength, and a "hole" appears.
One approach to solving the "halfwave hole"problem includes ensuring that, in spite of dispersion, the layers are halfwaves at precisely the same wavelengths. This approach strives to eliminate thickness errors as far as possible. An optical interference filter incorporating these corrections was disclosed in a report entitled "SLC Cesium Atomic Resonance Filter Interference Coatings," by H. A. Macleod et al., Final Report for Navy Contract No. N66001-82K-0187 (September 1983).
The present invention provides an alternate solution to the halfwave hole phenomenon.